This invention relates to electric motors in particular, but not exclusively to, variable speed brushless electric motors.
Variable speed electric motors are known prior art. In many motor applications audible noise is undesirable, and can become particularly noticeable at low motor speeds and when the electrical supply to such motors is varied during transitions from the original speed to the varied speed. Audible noise is also associated with energy loss. It is also known that speed variations of prior art motors are difficult to control because these motors are generally designed to operate at a fixed supply frequency. Speed control difficulties are most pronounced in prior art fractional and subfractional horsepower motors with designs which have been optimised or minimised for low manufacturing costs. It is however possible to improve the speed variation and control of these motors by electronic means. In addition mechanical means such as but not limited to gearboxes, belts, variators and electromagnetic couplings may also be incorporated to improve speed control. As a consequence, these improvements increase the cost and complexity of the complete motor assembly. Prior an motors have also been found to have energy efficiencies (defined as the ratio of mechanical output power to electrical input power) of 20% or less when operating at fixed speeds. The addition of speed control means reduces the energy efficiency still further.
It is one object of the present invention to provide a variable speed electric motor which is more efficient or audibly quieter at least when varying speeds, or to at least provide the public with a useful choice.
According to one aspect the invention provides an electric motor including means for determining a first signal associated with the rotor (when the motor is operational) and wherein said first signal is applied as an input signal to a power amplifier, which generates an output which is applied to windings of the motor to drive the motor, and whereby in use, audible noise associated with the motor is thus significantly reduced.
The invention may make use of a sense winding as described in our earlier New Zealand Patent application 501086 (the contents of which are incorporated herein by way of reference) to determine the first signal, or it may simulate this signal electronically without the need for a sense winding. As a further alternative the invention may derive an approximation of the first signal by sensing the voltage or current applied to the windings. In either case the first signal is applied to a power amplifier to create an output which is applied to the motor to drive the motor.
According to a second aspect the invention provides an electric motor including electronic back EMF sensing means, the electronic back EMF sensing means including voltage sensing means and current sensing means and rate-of-change of current sensing means adapted to sense a back EMF generated by a rotor of the motor wherein the sensed electronic back EMF is applied as an input signal to a power amplifier, which generates an output which is applied to windings of the motor as a drive voltage or current, and whereby in use, audible noise associated with the motor is thus significantly reduced.
According to a third aspect the invention provides an electric motor including sensing means to sense an approximation of the back EMF produced by rotation of the rotor by sensing the voltage or current applied to a motor winding, the sensing means providing a first signal which is applied as an input signal to a power amplifier, which generates an output which is applied to windings of the motor as a drive voltage or current, and whereby in use, audible noise associated with the motor is thus significantly reduced.
According to a fourth aspect the invention provides an electric motor including back EMF prediction means, the back EMF prediction means including a memory from which data relating to the motor are stored, the back EMF prediction means providing a first signal indicative of the motor back EMF, the first signal being applied as an input signal to a power amplifier, which generates an output which is applied to windings of the motor as a drive voltage or current, and whereby in use, audible noise associated with the motor is thus significantly reduced.
Preferably the electric motor is a radial flux permanent magnet motor. In the alternative the electric motor can be an axial flux motor or other type of direct current motor.
Preferably the motor is a brushless motor.
Preferably the rotor is an external rotor rotatable about a central shaft.
Preferably the rotor houses a plurality of spaced permanent magnets arranged in a radial pattern on an internal surface of the rotor.
Preferably the electronic back EMF sensing means includes current sensing means which detect the steady state value of the motor winding current in each motor phase.
Preferably the electronic back EMF sensing means also includes rate-of-change of current sensing means which detect the rate-of-change of motor winding current in each phase.
Preferably the electronic back EMF sensing means also includes voltage-sensing means which sense the voltage applied to the motor windings.
Preferably the electronic back EMF sensing means also includes a summing amplifier which allows the addition of signals from the current sensing means and the rate-of-change of current sensing means and the voltage sensing means, so that the output of the summing amplifier is analogous to the voltage field generated by the rotor of the motor.
Preferably the electronic back EMF sensing means also includes filtering circuits that can introduce a phase shift between the summing amplifier input voltages and the summing amplifier output voltage which is then applied to the power amplifier input.
Preferably the power amplifier means comprises power conversion means to adjust an available supply voltage to voltage levels suitable for driving the motor at the required speed controlled either directly or indirectly by the electronic back EMF sensing means.
In another aspect the invention resides in a method of reducing audible noise associated with an electric motor including the steps of:
1. Sensing a first signal associated with the voltage generated by a motor""s rotor from rotation of the rotor in use.
2. Amplifying and applying directly the first signal to windings of the motor as a drive voltage or current.
3. Controlling the drive voltage or current amplitude to control the motor whereby in operation audible noise is significantly reduced.
Preferably the electric motor is a radial flux permanent magnet motor. In the alternative the electric motor can be an axial flux motor or other type of direct current motor.
Preferably the motor is a brushless motor.
Preferably the rotor is an external rotor rotatable about a central shaft.
Preferably the rotor houses a plurality of spaced permanent magnets arranged in a radial pattern on an internal surface of the rotor.
Preferably the search coil means comprises a coil or coils wound coincidentally with one or more of the drive windings of the motor.
Preferably the electronic field sensing means includes a power amplifier that amplifies the search coil voltage. Preferably the electronic field sensing means also includes filtering circuits that can introduce a phase shift between the search coil voltage and the power amplifier output voltage which is then applied to the motor windings.
Preferably the power amplifier means comprises power conversion means to adjust an available supply voltage to voltage levels suitable for driving the motor at the required speed controlled either directly or indirectly by the electronic field sensing means.
In another aspect the invention resides in a method of reducing audible noise associated with an electric motor including the steps of:
1. Sensing, approximating or predicting a voltage or current generated by a motor""s rotor.
2. amplifying and applying directly the generated voltage or current to windings of the motor as a drive voltage or current,
3. controlling the drive voltage or current amplitude to control the motor whereby in operation audible noise is significantly reduced.